The present invention relates to can printing machines and more particularly to a mandrel for holding a can during the can printing operation.
Can printing machines, especially high speed continuous can printing machines, operate by the impingement of a rotating, image-carrying blanket wheel and an oppositely rotating can carrying mandrel wheel. The blanket wheel, which is at least as wide as the length of the cans, carries a series of wet ink images circumferentially spaced on its resilient periphery. The mandrel wheel carries a series of circumferentially spaced, rotatable shaft, or mandrel, assemblies, over which cans are fitted. The cans rotate on the mandrel wheel into registry and contact with the images and surface of the blanket wheel. The cans rotate about the mandrel to circumferentially receive the image from the blanket wheel. Each mandrel generally includes structure for removing cans from or drawing cans onto the mandrel shaft. This structure may be a hollow shaft extending through the center of the mandrel and controllably connected to vacuum and/or pressurized air lines.
Conventional thinking in the design of mandrels has dictated that a mandrel be rigidly positioned relative to the mandrel wheel in order to maintain alignment between the length of the can and the blanket wheel. Any misalignment would cause the image transferred to the can to be lighter towards the bottom or the top of the can. So that the outer surface of the mandrel may spin about its central mandrel axis, ball bearings generally are placed at axial ends of a central shaft and an outer mandrel sleeve.
The foregoing mandrel structure is well known in the art. U.S. Pat. No. 3,356,019 discloses a mandrel having an outer sleeve rotatably fixed to a mandrel shaft by ball bearings at axial ends of the mandrel sleeve. Similar mandrel structure is shown in U.S. Pat. No. 4,037,530, in connection with a mandrel trip mechanism which moves the mandrel away from the blanket wheel to prevent printing of the mandrel when no can is on the mandrel. In U.S. Pat. No. 3,388,686, three axially spaced ball bearings are used between the mandrel shaft and the sleeve. The mandrel shaft is adjusted into angular relationship with the blanket wheel to counteract deflection of the mandrel shaft by pressure of the printing blanket.
It can be seen, then, that drawbacks and limitations of prior art mandrel structures involve the alignment of the mandrel for proper printing by the blanket wheel. Misalignment may occur through normal manufacturing tolerances, improper alignment of the mandrel shaft, or deflection of the mandrel shaft. Another source of misalignment is slop or play in the other parts of the mandrel structure, particularly in relatively complex mandrel structures such as those employing a trip mechanism.